1. Field of the Invention
The present invention relates to an active noise controller and controlling method and, more particularly, to a three-dimensional active noise controller incorporated into a power generating system for private power generation or a transformer.
2. Description of the Related Art
Frequencies of audible sounds are in the range of 20 Hz to 20 kHz. Unwanted sounds unpleasant to human, among audible sounds, are called noises. There are various methods of controlling noises. For example, an active noise controller for controlling noise unidirectionally propagating through an exhaust duct uses an additional-sound-producing loudspeaker placed in the exhaust duct as an additional sound source. This active noise controller is called a one-dimensional active noise controller and has been prevalently used. The unidirectional active noise controller controls only sounds propagating past a position where the loudspeaker is placed.
The one-dimensional active noise controller is intended for suppressing only noises propagating through an exhaust duct, i.e., one-dimensional sounds, and is not intended for suppressing noises emitted by a sound source placed in an open space and three-dimensionally propagating in the open space. The one-dimensional active noise controller is capable of suppressing only part of sounds propagating through the exhaust duct and is incapable of suppressing sounds produced in other part of the exhaust duct.
Theoretically, the sound suppressing effect of a three-dimensional active noise controller intended for suppressing three-dimensionally propagating sounds can be enhanced by disposing the additional-sound-producing loudspeaker for producing an additional sound close to a noise source. Noise suppressing effect can be realized even at a position remote from the noise source. However, it is difficult to dispose the additional-sound-producing loudspeaker close to the noise source because the additional-sound-producing loudspeaker is affected by heat and vibrations generated by a device as a noise source or by a device disposed near the noise source. Since there is a technical trend of miniaturizing a system including the device as a noise source, it is difficult to secure a space available for installing the additional-sound-producing loudspeaker in the system including the device as a noise source, and hence the three-dimensional active noise controller is unable to suppress noises satisfactorily.
The present invention has been made in view of the foregoing problems in the conventional noise control apparatus and it is therefore an object of the present invention to provide an active noise controller capable of exercising a satisfactory noise suppressing effect without being affected by heat and vibrations generated by a noise source or a device disposed near a noise source.
According to the present invention, an active noise controller includes: an additional sound source that produces a sound of a frequency substantially equal to that of an unnecessary sound among sounds emitted by an unnecessary sound source to reduce the acoustic power of at least the unnecessary sound, and a duct connected to the additional sound source to conduct a sound produced by the additional sound source; wherein the duct has an opening through which the sound emitted by the additional sound source is radiated, and the duct is formed such that the sound emitted by the additional sound source and having a phase substantially opposite to that of the unnecessary sound is radiated through the opening into the vicinity of the unnecessary sound producing source to cancel out and reduce acoustic power of the unnecessary sound when the opening is at a distance not greater than about half the wavelength of the unnecessary sound from the unnecessary sound producing source.
A transformer provided with the active noise controller of the present invention includes a core, coils wound on the core, a tank holding the core and the coil therein, the additional sound source capable of producing a sound of a frequency substantially equal to that of a sound to be controlled to reduce the acoustic power of the sound among those produced by the core and the coils, and disposed apart from the tank, and the duct connected to the additional sound source to conduct the sound produced by the additional sound source to the tank and to send out the sound produced by the additional sound source to the vicinity of the tank. The distance between the tank and the opening is not greater than substantially half the wavelength of the unnecessary sound to be controlled. The phase of the sound sent out through the opening is opposite to that of the unnecessary sound to be controlled.
A power generating system provided with the active noise controller of the present invention includes a power generator, an internal combustion engine (hereinafter referred to simply as xe2x80x9cenginexe2x80x9d), the additional sound source capable of producing a sound of a frequency substantially equal to that of a sound to be controlled to reduce the acoustic power of the sound among those produced by the engine, and disposed apart from the engine, and the duct connected to the additional sound source to conduct the sound produced by the additional sound source to the engine and to send out the sound produced by the additional sound source to the vicinity of the engine. The distance between the engine and the opening is not greater than substantially half the wavelength of the unnecessary sound to be controlled. Pistons that move in cylinders formed in the engine produce sounds. The phase of the sound sent out through the opening is opposite to that of the unnecessary sound to be controlled.
Acoustic power expressed in watt is the amount of energy produced in a unit time by a sound source. Acoustic power level (dB) is represented by 10 log (I/I0), where I0=10xe2x88x9212 (w) is reference acoustic power; that is, acoustic power is a specific energy value representing the magnitude of noise. Sound pressure level (dB) is the pressure variation P (N/m2) of a sound wave at an observation point expressed by 20 log (P/P0) (dB), where P0 is reference sound pressure expressed by P0=2xc3x9710xe2x88x925 (N/m2).
For example, when the acoustic power level of a sound source is 80 dB, the sound pressure level at a position around the sound source is dependent on the distance between the sound source and the position; for example, the sound pressure level at one position is 70 dB and that at another position is 60 dB. If the acoustic power level of the sound source is reduced, the sound pressure levels at positions decreases accordingly.